The present invention relates to X-ray devices for the investigation of material structure, density and geometry of reflected surfaces by measuring reflected, diffracted or scattered radiation. These X-ray optical devices are especially useful for measuring polished surfaces with large reflective areas which are used in the electronics and the computer industry (wafers, memory discs), high precision mechanics and optics.
An X-ray reflectometer has previously been described [1] that has the following components: a source of polychromatic X-ray radiation; the means for X-ray beam collimation; a sample holder with provisions for rotating the holder around its own axis; an arm support with a means for rotation around an axis coinciding with the sample holder axis; a means for measuring the rotation angles of the sample holder and arm support; a crystal-monochromator; and a detector [1]. In this device [1], the monochromator is located between the X-ray source and the sample holder. Therefore, to switch the measurement to a new spectral region it is necessary to rotate the monochromator and other elements of this device corresponding a new Bragg angle. These elements are the X-ray source, and the means for making the measurements of the rotation angles of the sample holder, arm support etc., which are usually the parts of a precise opto-mechanical goniometer. The change in relative positions of the elements of the X-ray device causes misalignment of the device and requires accurate adjustment, which is a time consuming procedure, which takes a much longer time than that needed for the actual measurement of the sample parameters. For this reason, the measurements of spectral dependence of real and imaginary parts of the refractive index and scattering diagram are inefficient and poorly reproducible. These measurements are regularly used for the determination of density and composition of the bulk and surface layer of a sample. The above mentioned shortcomings of this device [1] reduce considerably the accuracy and reliability of the obtained data.
Also known in the art is an X-ray reflectometer containing a source of polychromatic X-rays, a sample holder with provisions for rotating the holder around its own axis, an arm support for which the rotation around the same axis is provided, a means for measuring the rotation angles of the sample holder and arm support, crystal monochromators, a crystal analyzer and a detector [2]. The crystal-monochromator is placed between the radiation source and the sample holder, and the crystal-analyzer is placed behind the sample holder on the path of the X-ray beam. A working region of the spectrum is cut out from the primary radiation by means of a monochromator, and the angles of reflection of the reflected or scattered radiation in respect of the direction of a primary monochromatic beam are measured by means of the rotation of the crystal-analyzer around its own axis. A perfect crystal whose diffraction angle does not change due to displacement of a radiation region is used as an analyzer. This makes it possible to eliminate the dependence of angular measurements on the position and dimensions of a sample in a beam. However, the installation of the second crystal makes the transmission of X-ray radiation considerably lower. Moreover, the changing of the spectral range (similar to the device described in reference [1]) requires long and complex adjustment.
In yet another X-ray reflectometer [3], the reflectometer comprises a source of polychromatic X-rays, a means of collimating of an X-ray beam, a sample holder, an arm support, a means of rotation of said sample holder and said arm support about a predetermined axis, tunable to a predetermined spectral band, a means of monochromatisation and detection of scattered, reflected or diffracted X-rays positioned on said arm support.
The main drawback of this arrangement is the random errors that are generated in measurements of different parts of the spectrum. These errors are due to the fact that such measurements, which are made by means of the device [3], should be performed successively after tuning the device for a new part of the spectrum. As a result, the conditions of data recording are changed due to the drift of the electric parameters of an X-ray source, the detector, and data processing channel during adjustment and repeated measurements. As a rule, this leads to uncontrollable errors connected with variations of the X-ray spectrum, the amplitude of the detector pulses, the level of noise, and the gain coefficient of the data processing channel. The other source of error is the non-reproducibility of the primary angular coordinates of the sample surface in respect to the beam after adjustment to a new part of the spectrum. The angular error is mainly due to the uncontrollable displacement of the sample during its repeated input into an X-ray beam, and the finite accuracy of angular measurements by means of a goniometric device.
In the described device [3], the rotation of only finite elements of the X-ray reflectometer (monochromator and detector) is needed to adjust the device to a new part of the spectral region. However, if a narrow part of the X-ray spectrum has been chosen, then it will take a considerable amount of time to adjust the monochromator and the detector. The total time needed for control and measurement of a single sample is made up of the measurement time (tm) in each part of the spectrum, the adjustment time (ta) of the measurement system, and the definition of initial angular coordinates (tk). Normally, ta+tk&gt;&gt;tm. Therefore, the efficiency in which measurements are made with such an X-ray reflectometer [3] is quite low.